Dimmers of various types are well known in the prior art. Mechanical rheostats must dissipate considerable wasted energy in the form of heat and must be closely coordinated in resistive value with the load being controlled in order to achieve optimal dimming effect. SCR dimmers are disadvantageous in that two devices in combination with an elaborate triggering arrangement are required in full wave AC applications. The triac has been the device of choice in prior art line voltage AC dimmers due to its inherently high efficiency and load independence. Since triacs require a relatively high trigger voltage on the order of 35 volts, their prior art usage has been largely limited to line voltage AC applications. The primary advantage of these prior art triac-based line voltage AC dimmers is that they may be easily configured in series with an incandescent lighting load as a complete controller and load circuit. In this configuration, the dimmer derives operating power via loop interaction with the load it controls to thereby achieve the same two-wire hookup as a toggle switch, which it often directly replaces. Its high efficiency and minimal heat dissipation facilitate installation in a confined space, such as a wall electrical box that may be conveniently accessed. The principal disadvantage of prior art triac-based line voltage AC dimmers is the generation of a relatively high level of undesirable radio frequency noise that results from the inherently rapid switching time of the triac. Since triggering of these prior art dimming circuits does not occur until the AC voltage reaches at least 35 volts, it is only possible to achieve a maximum brightness level of about 90% of an incandescent lighting load, as compared to about 95% in the case of an SCR-based dimmer and 100% when employing a direct switching device. Yet another disadvantage of triac-based line voltage AC dimmers is the potential for light flickering and unwanted extinguishing of the incandescent lighting load at or near the maximum dimming level.
Low voltage AC lighting systems are often preferred over line voltage AC lighting systems in the interest of increased safety, for example, in outdoor installations such as architectural lighting or swimming pool lighting. Another reason for using low voltage AC lighting is to take advantage of the superior color rendering characteristics, longer rated bulb life, and superior light beam concentration attributed to low voltage AC incandescent lighting. Although AC line voltage is generally considered safe in outdoor applications when proper wiring procedures are followed and a safety device such as an isolation transformer and/or ground fault interrupter is added, voltage levels below 50 volts are considered non-shocking and are therefore often preferred in wet environments to eliminate even the slightest possibility of electrical shock.
A standard step-down transformer in a protected location is typically used to reduce AC line voltage to 12 or 24 volts in order to power any of a variety of standard low voltage lighting fixtures in one or more unprotected locations. A major drawback of low voltage AC lighting as compared with line voltage lighting is that it requires proportionally more current to obtain the same level of illumination, thereby placing an increased strain on current-carrying components. For example, in a 12-volt AC system, a maximum of only about 100 watts of power can be delivered through a 16-gauge conductor while staying within the electrical wiring code.
Dimming of low voltage AC incandescent lighting is often desired, for example, to achieve a more dramatic effect from a particular architectural lighting scheme. One prior art approach for dimming low voltage AC incandescent lighting combines a line-voltage AC dimmer driving the primary side of a transformer with one or more low voltage incandescent bulbs dimmed from the secondary. However, a triac-based line voltage AC dimmer is an acknowledged poor device for driving the inductive load presented by the primary side of a transformer because the inherent fast switching time of the triac induces relatively high amplitude voltage spikes and current surges. Some combination of dimmer buzz, fixture buzz, lamp flickering, interaction between circuits, radio frequency interference, and possible damage to the dimmer is the frequent result of such an arrangement.
Another prior art low voltage AC dimmer is designed for driving the line side of a transformer, which may incorporate a third wire bypass to decouple spikes and surges as well as a shutdown arrangement in the event of overheating, which may be caused by losing the secondary load, for example. In order to assure proper operation and matching of components, a packaged system that includes transformer, fixture, dimmer, and bulbs is usually offered. Such packaged systems are disadvantageous in that the light bulbs are not individually dimmable, and the combination is nowhere near as adaptable to specific user needs as may be possible through the use of separate components.
Yet another prior art approach for dimming low voltage AC incandescent lighting is to first convert the low voltage AC waveform into a pulsing DC waveform using a standard rectifier, and then apply the resulting waveform to any DC dimmer known in the art. However, this method is suboptimal in that at low voltages, a standard rectifier dissipates a sizable percentage of the total incoming power as heat (about 10 percent at 12 volts AC) even before dimming is applied, thereby limiting siting options and achievable power delivery levels. A stand-alone, ventilated circuit box might allow power delivery beyond the 100-watt range, although the increased cost would probably limit use of this arrangement to specialty applications. This prior art approach is further disadvantageous in that filtering of the pulsing DC waveform may be required for proper operation of the dimmer, and two-wire implementation of the dimmer is unlikely.
It would therefore be advantageous to provide a two-wire triac-based low voltage AC dimmer adapted for configuration in series with a typically incandescent lighting load. In accordance with the illustrated preferred embodiment of the present invention, a step-down transformer provides a low voltage AC waveform, and a separate, electrically isoated loop circuit utilizing a step-up transformer converts a portion of the low voltage AC waveform into a corresponding high voltage AC waveform used to trigger the gate of a triac by means of any of a number of well known phase delay or special effects circuits. The remaining portion of the low voltage AC waveform is selectively gated without conversion through the power terminals of the triac to achieve variable power delivery to the incandescent load. An on/off arrangement that serves to lock the triac in its blocking state may be incorporated in the present low voltage AC dimmer. The present low voltage AC dimmer permits the use of a less expensive and more effective radio frequency filter capacitor.